Your furnace utilizes various components that work together to provide cozy warmth throughout your home during the colder months. Therefore, when one or more of the furnace's components are faulty, it can result in the furnace blowing cold air.
Some of the most common issues that result in a furnace blowing cold air include:
If your furnace continuously runs or the unit only blows out cool air sometimes, it could simply be that your thermostat is on the wrong setting.
Turn the thermostat setting to AUTO. When the fan setting is set to ON, it causes the furnace to blow out air even though the furnace is not producing heat. This can result in the furnace blowing cold air.
However, if you set the thermostat to AUTO, it will only blow out warm air.
A faulty pilot light, including a pilot light that doesn't come on or a pilot light that doesn't stay on, can also cause your furnace to blow out cool air.
Newer furnaces utilize an electronic ignition, which can cause problems if it is not adjusted properly.
Inspect the electronic ignition, and then have it adjusted or replaced if needed to restore proper function.
A dirty filter obstructs airflow over the unit's heat exchanger. This causes the furnace to run longer until eventually, it runs hot.
When the furnace gets too hot, it trips the high-temperature limit switch. This forces the burners to shut off, so the heat exchanger doesn't get damaged. However, the blower continues to blow to keep the furnace at a safe level, which can result in the furnace blowing cold air.
If you notice that the furnace blows out warm air and then cool air, or it stops blowing out air altogether after a while, it could be that a dirty filter is causing the furnace to overheat.
Shut off power to the furnace, and then inspect the filter. If the filter is dirty, then clean it or replace it, and then reset the system.
If your furnace is a newer high-efficiency model, and you notice water flooding around the unit, it could be due to a blocked condensate line.
When the condensate line is blocked, typically by dirt, dust, mold, or ice, it causes water to back up into the unit. This causes the furnace switch to shut off to prevent water damage.
A broken condensate pump can also cause the condensate line to overflow.
Cracks, holes, and gaps in the ducts allow cold air from the attic or crawlspace to enter into the ducts, which can make it feel like your furnace is blowing out cool air.
Dirt and debris accumulated on the inside of the ducts can also hinder airflow.
Inspect the ducts in the attic or crawlspace and around the unit for cracks, leaks, and gaps in the ductwork. Afterward, have a professional technician repair any openings if needed.
A furnace blowing cold air can be caused by various issues with your furnace's components, many of which you can address yourself. However, for more serious issues with your furnace blowing cold air, contact a technician who is highly knowledgeable and skilled in the inspection and repair of a furnace blowing cold air to prevent damaging your system.